1. Field of the Invention
The invention relates to a filtration method of colloid solution, specifically to a filtration method of a colloidal solution containing very fine substances having diameters less than 0.15 μm.
2. Description of the Related Art
It is critical for corporations operating in the 21 st century to reduce industrial wastes, to classify industrial wastes so that wastes can be recycled and not to release industrial wastes into the environment from an ecological standpoint.
Although various terms are employed to denote aqueous wastes such as sewage, wastewater and wastes, all aqueous and chemical solutions containing substances that require removal will hereinafter be generally called wastewater. The substances are generally removed from wastewater by expensive filtration devices so that the wastewater can be reused and that the separated substances or filter out substances are treated as industrial wastes. Water cleaned to satisfy environmental regulations by filtration is returned to natural environments such as rivers and seas, or reused.
However, due to the cost of filtration and associated running costs, it is very difficult to adopt these devices, which results in environmental problems.
As can be understood from the above, wastewater treatment technology is a very important issue from the standpoint of both environmental protection and resource recycling. The system with low initial cost and low running cost is keenly desired.
As an example, wastewater treatment in the semiconductor manufacturing field will be discussed below. In grinding a plate-like object made of metals, semiconductors, and ceramics, liquids such as water are showered over grinding jigs and the plate-like object in order to prevent temperature rises in grinding jigs, improve lubricity, and prevent grinding swarfs and machining chips from adhering to the plate-like object.
A typical practice is to use pure water for the shower in dicing or back-grinding semiconductor wafers, i.e., plate-like semiconductor members. On a dicing device, the showering is performed by forming a flow of pure water over semiconductor wafers or arranging a water nozzle to eject pure water on the semiconductor wafer in order to prevent temperature increase in the dicing blade or dicing chips from adhering onto the wafer. In thinning the thickness of the wafer by back-grinding, pure water is also used for the same reason.
Wastewater contaminated with grinding swarfs or polish swarfs discharged from a dicing device or back-grinding device is cleaned by filtration before it is returned to the environment, while wastewater concentrated as a result of reuse is collected.
As for the current semiconductor manufacturing process, the method of treating wastewater containing substances (debris) made mainly of Si can be divided into two methods, i.e., a coagulating precipitation method and a method of combining filtration and centrifugation.
In the coagulating precipitation method, coagulants such as PAC (poly aluminum chloride) and Al2(SO4)3 (aluminum sulfate) are mixed with the wastewater to cause reactions with Si, and reactants thus formed are removed as a means of filtration.
In the latter method in which filtration is combined with centrifugation, the wastewater is first filtrated to produce a condensate, which is then centrifuged to remove silicon debris as sludge, producing cleaned water to be returned to the environment or reused.
For example, as shown in FIG. 12, the wastewater generated during the dicing process is collected into a raw water tank 201 and fed to a filtration device 203 via pump 202. Filtration device 203 is equipped with a ceramic type or organic type filter F, so that the filtered water is fed to a collection water tank 205 via piping 204 to be reused or released into the environment.
In the meantime, the filter F of filtration device 203 eventually becomes clogged so that it is periodically cleaned. For example, a valve B1 on the raw water tank 201 side is closed while opening a valve B2 and a valve B3 to clean the filter F with the water reverse-fed from collection water tank 205. The wastewater thus generated containing a high concentration of Si debris is returned to raw water tank 201. The concentrated water from a concentration tank 206 is fed to a centrifuge 209 via pump 208 and is separated into sludge and separation liquid by centrifuge 209. The sludge mainly made of Si debris is collected in sludge collection tank 210 and the separated liquid is collected into a separation liquid tank 211. The wastewater from separation liquid tank 211 which collects the separation liquid is fed to raw water tank 201 via pump 212.
These methods are also used for collecting debris generated during the process of grinding bulky solids or a plate-like object made primarily of metals such as Cu, Fe, and Al, or bulky solids or a plate-like object made primarily of inorganic substances such as ceramics.
CMP (chemical-mechanical polishing) is a newly introduced semiconductor processing technology.
The CMP technology provides the following features:
(1) Realization of a flatter device surface.
(2) Realization of a structure embedded with a material different from the substrate.
Feature (1) allows microscopic patterns to be accurately formed using lithographic techniques. When combined with the Si wafer pasting technique, it also allows realizing three dimensional ICs.
Feature (2) allows embedded structures. For the multi-layer wiring of ICs, the tungsten (W) embedding technique has been used. This method is to embed tungsten into grooves formed in interlayer films by the CVD method, which is then flattened by etching back the surface. More recently, it is flattened by the CMP method. The application of this embedding technique includes the Damascene process and element separation.
The CMP technology and its applications are described, for example, in “CMP Science” published by Science Forum.
The CMP mechanism will be described briefly. As shown in FIG. 13, a semiconductor wafer 252 is placed on a grinding cloth 251 carried on a rotating platen 250 to lap, grind and chemical etch together while pouring abrasive (slurry) 253 to remove structures associated with the surface roughness of wafer 252. The chemical reaction due to the solvent contained in abrasive 253 and the mechanical grinding action due to the grinding cloth and the abrasive grinding stones contained in the abrasive provides planarization function. Grinding cloth 251 can be foamed polyurethane, unwoven fabrics, and the like, while the abrasive can be abrasive grinding stones such as silica and alumina mixed with water containing pH adjusters, and is normally called slurry. Wafer 252 is lapped against grinding cloth 251 by rotating it and applying a certain pressure, while pouring slurry 253. A dressing unit 254 is provided to constantly redress grinding cloth 251 to maintain the grinding capability of grinding cloth 251. The drawing also shows motors 202, 208 and 212 as well as belts 255, 256 and 257.
The above mechanism is constructed as an integrated system as shown in FIG. 14. The system can be divided primarily into a wafer cassette loading/unloading station 260, a wafer transfer mechanism unit 261, a grinding mechanism unit 262 as described in FIG. 13, a wafer cleaning unit 263, and a system controller that controls all of the above.
A cassette 264 carrying at least one wafer is placed on wafer cassette loading/unloading station 260, where a wafer carried by cassette 264 is taken out. Next, wafer transfer mechanism unit 261, for example, a manipulator 265, holds the wafer, and places it on rotating platen 250 provided in grinding mechanism unit 262 so that the wafer can be planarized by the CMP technology. When the planarization process is finished, the wafer is transferred to wafer cleaning unit 263 by manipulator 266 to be cleaned. The cleaned wafer is returned to wafer cassette 266 to be stored.
The amount of slurry used for one cycle is typically about 500 cc to one liter/wafer. In addition, pure water is consumed in grinding mechanism unit 262 and wafer cleaning unit 263. All of these liquids are finally joined at the drain, and wastewater of approximately 5 to 10 liters/wafer is discharged per each planarization process. In the case of a triple layer metal, a total of seven planarization processes are performed for planarizing the metals and interlayer insulation films, generating seven times as much waste (7×5–10 liters) before one wafer is completed.
Thus, a large amount of slurry diluted by pure water is discharged when the CMP device is used. This wastewater has been treated by the coagulating precipitation method because the wastewater is a colloidal solution.
However, the coagulating precipitation method uses chemicals as coagulants. It is very difficult to estimate the right amount of chemicals for a complete chemical reaction, thus an excessive amount of chemicals is loaded and the chemicals remain unconsumed at the end. On the other hand, if the amount of chemicals is not sufficient, some substances remain in the solution. If the amount of chemicals is excessive, the chemicals remain in the supernatant liquid. If such wastewater is intended to be reused, the chemicals remaining in the filtered liquid present a problem that they cannot be used for any application that prohibits inclusion of chemicals.
Flocks that are reactants of chemicals and substances look like and float like seaweed. The condition for forming flocks is attained by a severe pH control and requires an agitator, a pH measuring device, a coagulant injection device and a control device to control the process. In order to stabilize flocks and allow them to precipitate, a large settling tank is necessary. For example, to achieve a 3 cubic meter (m3)/hour treatment capacity, a tank with a 3 meter diameter and a 4 meter depth (a settling tank of an approximately 15 ton capacity) is required, which in turn requires a substantial space of approximately 11×11 meters to install the entire system.
Moreover, some of the flocks remain floating without precipitating, and are more likely to flow out of the tank, and it is difficult to collect all of them. Thus, the system is subject to problems such as a large system size, high initial cost, difficulty in water reuse and high running cost due to the use of chemicals.
On the other hand, a combination system of filtration and centrifugation with a 5 cubic meter (m3)/hour treatment capacity as shown in FIG. 12 uses a filter F (a filter called an UF module using a polysulfone type fiber or a ceramic filter) for filtration device 203 so that water reuse is possible. However, since filtration device 203 requires four filters F that have a short life, it is necessary to replace four of these expensive filters, each costing approximately 500,000 yen, at least once a year. Moreover, pump 202 placed ahead of filtration device 203 tends to develop clogging, causing a high load on the motor, since the filter F is based on a pressurized type filtration method. Thus, pump 202 requires a high capacity. Furthermore, approximately two thirds of the wastewater that passes through the filter F is returned to raw water tank 201. The wastewater that contains substances attacks the inner walls of pump 202 that feeds the wastewater so that the life of pump 202 is shortened considerably.
In summary, the problem with this method is an extremely high running cost due to high electric power consumption and replacements of the pump P and the filter F.
Furthermore, CMP consumes an amount of wastewater much higher than the dicing process. The slurry is distributed in the fluid in a colloidal state and does not subside easily due to its Brownian movement. In addition, the particle size of abrasive grinding stones mixed in the slurry is as fine as 10–200 nm in diameter. Therefore, if the slurry including such fine particles is filtered using a filter, the pores of the filter become very quickly loaded with abrasive grinding stones, causing clogging often and making it difficult to process a large amount of wastewater.
As can be seen from the above explanation, the filtration device for wastewater used for removing substances that affect the global environment or for reusing filtered fluids or separated substances is often a substantial system of a high initial and running costs. Therefore, these conventional wastewater treatment devices are hardly useful in practical applications.
It has been a general belief that a filter membrane having pores smaller than sub-micrometer particles is required in order to remove the sub-micrometer particles of 0.15 μm or smaller, such as abrasive grinding stones contained in CMP slurry. However, since such a filter does not exist, a common knowledge has been that it is impossible to filter such a liquid.